In the art of container manufacture, the term "container finish" generally refers to that portion of the container that defines the container mouth. In a bottle, for example, the finish includes that portion of the container neck having threads and/or shoulders for receiving the container cap, as well as the upper surface of the neck surrounding the container mouth against which the cap seats. It is important that the container finish be properly manufactured so that a cap may be affixed thereto to seal the container cavity against leakage and escape of carbonation during handling and storage.
Conventional technology for mass production of glass or plastic containers involves forming the containers in a multiplicity of molds. It has heretofore been proposed to employ optical scanning techniques for inspecting such containers for variations that affect optical characteristics of the container. In U.S. Pat. Nos. 4,378,493, assigned to the assignee of the present application, there are disclosed methods and apparatus in which glass containers are conveyed through a plurality of stations where they are physically and optically inspected. At one inspection station, a glass container is held in vertical orientation and rotated about its vertical axis. An illumination source directs diffused light energy through the container sidewall. A camera, which includes a plurality of light sensitive elements or pixels oriented in a linear array parallel to the vertical axis of container rotation, is positioned to view light transmitted through a vertical strip of the container sidewall. The output of each pixel is sampled at increments of container rotation, and event signals are generated when adjacent pixel signals differ by more than a preselected threshold level. An appropriate reject signal is produced and the rejected container is sorted from the conveyor line.
U.S. Pat. No. 4,584,469 discloses a device for detecting vertical checks in the sidewall of a glass container. A "vertical check" is a minute planar crack that extends into the container wall and is parallel to the container axis. (Directional terms such as "vertical" assume vertical orientation of the container axis with the mouth opening upwardly, as is typical in the container inspection art.) A light source is positioned to one side of the container to direct light energy onto the container wall from a direction perpendicular and lateral to the container axis. An optical detector is positioned to receive an image of the illuminated portion of the container wall from a direction perpendicular to the illumination and container axes. As the container is held in position and rotated about its axis, a planar check will eventually be rotated into a position to reflect light from the source onto the detector. Thus, the vertical check is detected as a bright spot on what is otherwise a normally grey or dark background viewed by the detector.
Two problems are presented by such prior art apparatus for detecting vertical checks in the container sidewall. First, since the light source is tightly focused at the container sidewall and of fairly limited angular extent, light from the source reflected by the check will not be directed onto the detector if the check is other than nearly radially oriented. Second, the detector senses presence of a radially oriented check, but cannot resolve details of the check such as size, and therefore cannot discriminate from other commercial variations that reflect light onto the camera. It is therefore a general object of the present invention to provide a method and apparatus for detecting vertical checks in the finish of translucent containers, such as glass bottles, that include facility for detecting non-radially oriented checks over a wide angular range with respect to the container axis, are capable of resolving size and orientation of the check, distinguishing acceptable commercial variations in finish quality from unacceptable checks, and/or are capable of distinguishing between vertical checks and dust, mold seams or other manufacturing variations.